The present invention relates to a heat-treating method and apparatus which can produce rails of a variety of strength levels by cooling the rails from a temperature range in austenite range after hot rolling or after a heating for the purpose of the heat treatment.
The current trend for heavier axle load and higher speed in railroad transportation has caused a tendency of rapid wear and fatigue of the rail heads, which in turn has given rise to the demands for rails having higher anti-wear and anti-damage properties, and for rails of various levels of strength from medium level strength (Hv&gt;320) to high level strength (Hv&gt;360).
Such a demand has been met, as confirmed through studies, by steel rails having fine pearlite structure. It is well known that this type of rails exhibit superior anti-wear and anti-damage properties.
An alloy steel rail as a prior art is disclosed in Japanese Unexamined Patent Publication No. 140316/1975. This rail is made of an alloy steel which is obtained by adding elements such as Si, Mn, Ni, Cr, Mo and Ti to a carbon steel, and is used in an as-rolled state. Japanese Examined patent Publication No. 23885/1980 discloses another prior art rail of a kind described below. This rail does not contain any alloy elements but the head portion of this rail is re-heated to a high temperature and is cooled from a predetermined temperature region with a control of the cooling rate throughout a certain temperature range.
The known rails, however, suffer from the following disadvantages.
Namely, the rail of the first mentioned type with its composition controlled by addition of alloy elements, intended for use in an as-rolled state, necessitates a large amount of alloy elements. These elements are generally expensive so that the cost of production of the rail is raised undesirably.
The rail of the second-mentioned type is produced typically by directing a cooling medium such as water and gas to the head of the rail material which has been heated to a high temperature, thereby forcibly cooling the rail head from the high temperature. This method is effective only when rails of a given strength are to be produced, and is not suited to the case where rails of a variety of strength levels are to be obtained. Although, in the production of this type of level, contents of carbon and other alloy elements added to the material fluctuate in the step of steel making which carbon and alloy elements substantially determine the level of the strength of the rails, it has been impossible to compensate for the fluctuation with the result that rails of desired strength level can not be obtained in the prior art.